md: use new plugging interface for RAID IO.

md/raid submits a lot of IO from the various raid threads.
So adding start/finish plug calls to those so that some
plugging happens.

Signed-off-by: NeilBrown <neilb@suse.de>
diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
index 2da83d5..c8e5dac 100644
--- a/drivers/md/raid10.c
+++ b/drivers/md/raid10.c
@@ -1640,9 +1640,11 @@
 	conf_t *conf = mddev->private;
 	struct list_head *head = &conf->retry_list;
 	mdk_rdev_t *rdev;
+	struct blk_plug plug;
 
 	md_check_recovery(mddev);
 
+	blk_start_plug(&plug);
 	for (;;) {
 		char b[BDEVNAME_SIZE];
 
@@ -1716,6 +1718,7 @@
 		}
 		cond_resched();
 	}
+	blk_finish_plug(&plug);
 }